Numerous examples have appeared in recent years of the execution of clear coatings on metals (typically aluminum and stainless steel) for the purpose of producing a film which resists fingerprints, corrosion, and weathering and which simultaneously exploits the glossy appearance of the metal substrate. Clear coating in such cases has normally required a different type of undercoating or priming treatment from that optimized for underlying typical colored paints.
Colored paints generally contain sufficient pigment to hide the appearance of any priming or undercoating treatment used underneath them, so that the aesthetic impact of the substrate color generated by the undercoating treatment is a matter of little concern. However, in the case of clear coatings, the color of the undercoating treatment directly affects the post-painting appearance. In addition, it is highly desirable in the case of clear coatings to exploit the metallic texture of the coated metal. As a consequence, the undercoating treatment normally should be colorless or only very weakly colored. Moreover, clear coatings and color clear coatings usually impose limitations on the additives (pigments and the like) and employ special resins (e.g., fluorine containing resins) in contradistinction to colored paints, and as a result the known undercoating treatments in some cases may not provide a satisfactory adherence, corrosion resistance, and weather resistance.
Phosphating treatments and chromate treatments have been heretofore employed as undercoating treatments for metals which are to be painted.
Phosphating treatments are associated with the following two problems: limitations on the treatable metals, and reduction of the metal gloss due to the formation of a conversion film on the metal surface.
Chromate treatments are typically divided into the following 3 categories: reaction-type chromate treatments, electrolytic chromate treatments, and application- or coating-type chromate treatments. Reaction-type chromate treatments suffer from limitations on the treatable metals and from the general inability to avoid the coloration problem. Thus, when the associated coloration is reduced by limiting the film weight, the corrosion resistance and paint adherence become unsatisfactory because the film weight is then no longer adequate for these purposes.
Limitations on the applicable metals are not encountered in the case of electrolytic chromate treatments, but this type of treatment has not generally provided a satisfactory corrosion resistance. Application-type chromate treatments are not limited with regard to applicable metals and provide a relatively good post-painting performance with typical colored paints. However, conventional appli-cation-type chromate treatments cannot avoid the coloration problem, and conventional application-type chromate treatments often give an unsatisfactory paint adherence with the fluorine-type paints used for contemporary clear coatings. This led to an examination of the application-type chromate treatments which have been disclosed in the patent literature.
Included among these are, for example, the treatments proposed in Japanese Patent Application Laid Open [Kokai or Unexamined] Number 62-270781 [270,781/87] and Japanese Patent Application Laid Open Number 63-270480 [270,480/88]. Japanese Patent Application Laid Open Number 62-270,781 does not give a satisfactory basis for the clear coating art; coloration is still a problem because it employs a (trivalent chromium)/(hexavalent chromium) weight ratio in the range of 0.2 to 1.0. Furthermore, its paint adherence remains unsatisfactory. On the other hand, while Japanese Patent Application Laid Open Number 63-270480 is silent with regard to clear coatings, it nevertheless provides improvement with regard to post-treatment appearance and post-painting performance. However, this method places emphasis on obtaining a transparent whiteness for the post-treatment appearance in the case of no subsequent painting, and it requires the addition of an inorganic colloidal compound (silica sol or alumina sol). As a result, problems still remain with the paint adherence and the long term durability after painting.
In addition to these processes, other tactics include the application of paint after only a degreasing step and the use of silane coupling agent in the undercoating treatment (an example of the latter is Japanese Patent Publication Number 63-35712 [35,712/88]). No coloration problem is encountered in either approach, but the former approach suffers from an unsatisfactory paint adherence, corrosion resistance, and weather resistance while the latter approach suffers from an unsatisfactory corrosion resistance and weather resistance, although it does have an effect on the paint adherence.